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WkntJs G*?t*ticsl
Guide
for Reading
After you read the following
sections, you will be able to
1-1
History of Genetics
I Describe how traits are
passed from one
generation to another.
B Explain the difference
between dominant and
recessive traiis.
1-2 Principles of Genetics
I State the law of
segregation and the law
of independent
assortment.
!
1-3
Explain what is meant by
incomplete dominance.
Genetics and Probability
E Relate the law of
probability to the study
of genetics.
E Describe how Punnett
squares can be used to
predict the results of
genetic crosses.
Do you have a cat, or do you have a friend who
has a cat? You probably know that there are many
different kinds of cats. Some cats have long, soft
hair and fluffy tails. Others have short, curly hair
and skinny tails. There is even one kind of cat that
appears to have no hair at all! Despite these, and
many other, variations, you can still recognize these
animals as cats. \{hy is this so? What makes one cat
different from another and yet still recognizable as a
member of the cat family? The answer can be found
in the science of genetics.
The study of genetics explains why one cat is
different from all other cats and also why a cat is
different from..a human or a maple tree.'-All living
things, including cats, resemble their parents. But
each individual also has certain unique characteristics that make it different from every other living
thing on Earth. As you will learn in this chapter, the
history of genetics is a fascinating story of mystery
and discovery. The story begins with one man in a
garden more than 100 years ago. . . .
Journal AsJi-uityYou and Your Wortd Have you ever seen a litter of kittens or puppies?
Did all the kittens or puppies look exactly alike? ln your journal, describe
how they were alike and how they were different. Do you have any idea
why they look as they do?
4 fn"t"
kittens may have different colors and markings, but they atl look like cats.
Guide for Reading
Focus on this question as
you read.
* What is meant by the term
genetics?
Figure 1-1 Gregor Mendel is
shown in his garden studying how
traits are passed on from parents
to offspring. What organisms did
Mendel study?
t *f History
of Genetics
The history of genetics began with a monk
named Gregor Mendel working in the garden of a
small monastery in eastern Europe. Mendel, whose
parents were Austrian peasants, was born in 1822.
He entered the monastery at the age of 2l and was
ordained a priest 4 years later. In 1851, Mendel was
sent to the University of Vienna to study science and
mathematics. After he left the university, Mendel
spent the next 14 years working at the monastery
and teaching at a nearby high school. In addition to
teaching, Mendel also looked after the monastery
garden. Here he grew hundreds of pea plants.
Mendel experimented with the pea plants to see if
he could find a pattern in the way certain characteristics Were handed down from one generation of pea
plants to the next.
Mendel chose pea plants for his experiments for
several reasons. Pea plants grow and reproduce
quickly. So he knew that he could study many generations of pea plants in a short time. Mendel also
knew that pea plants had a variety of different characteristics, or traits, that could be studied at the same
time. Pea plant traits include how tall the plants
grow, the color of their seeds, and the shape of their
seeds. Mendel could study all of these traits (as well
as other traits) in the same experiment. In addition,
pea plants could be crossed, or bred, easily.
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Figure 1-2 shows what the flowers of Mendel's
pea plants look like. As in most flowering plants, the
flowers of pea plants contain stamens, or male
reproductive structures. Stamens produce pollen,
which contains male sex cells, or sperm cells. The
flowers also contain the female reproductive structure, called the pistil. The pistil produces the female
sex cell, or egg cell. When pollen lands on top of the
pistil of a flower, pollination occurs. Poltination produces seeds for the next generation of pea plants.
Usually, a pea plant pollinates itself. This type of
pollination is known as self-pollination. In self-pollination, pollen from the stamen of one flower lands
Figure 1-2 Stamens Produce
pollen, which contains sperm cells.
The pistil produces eggs. As a
result of pollination, fertilized eggs
develop into seeds. When Planted,
a seed grows into a new Pea
plant.
on the pistil of the same flower or on the pistil of a
different flower sn\,the same plant. But Mendelfound that he could'transfer pollen from the stamen
of one flower to the pistil of another flower on a
different plant. This type of pollination is known as
cross-pollination. By using cross-Pollination, Mendel
was able to cross pea plants with different traits.
Although Mendel did not tealize it at the time,
his experiments would come to be considered the
Tulips Are Better Than One,
p.1 04
,,-\
Pollen
(male sex cells)
Figure 1-3 The pollen from one
ftower is transferred to the pistil of
another flower on a different Pea
plant. What is this Process called?
beginning of the science of genetics (juh-rvnnr-ihks).
For this reason, Mendel is called the Father of
Genetics. Genetics is the study of heredity, or the
passing on of traits from an organism to its
Genetic Scientists
offspring.
You may be interested in
finding out more about some
of the people listed below.
They are all scientists who
fulendel's Experiments
have contributed to the
study of genetics and heredity. The library has many resources to help you find out
about these people and their
work. Choose one of these
people and find out what she
or he discovered about heredity and genetics. Present
your findings in a report.
Rosalind Franklin
Jarnes D. Watson
Martha Chase
Jacques Monod
Barbara McClintock
Figure 1-4 ln Mendel's first
experiment with pea plants,
crossing two short plants resulted
in offspring that were all short as
well. What did Mendel call these
short plants?
Mendel began his experiments by first crossing
two short pea plants (pea plants with short stems).
He discovered that when he planted the seeds from
these pea plants with short stems, only shortstemmed plants grew. In other words, members of
the next generation of short-stemmed plants were
also short-stemmed. This result was what he, and
everyone else at that time, expected. New generations of plants always resembled the parent plants.
Mendel called these short plants true-breeding
plants. By true-breeding plants, Mendel meant those
plants that always produce offspring with the same
traits as the parents.
In the experiments that followed, Mendel tried
crossing two tall pea plants (pea plants with long
stems). He wondered if the tall pea plants would also
be true-breeding. To his surprise, he found that tall
pea plants would not always be true-breeding. Some
tall pea plants pioduced all tall plants. However,
other tall pea plants produced mostly tall and some
short pea plants. This result was different from the
cross between the short pea plants, which produced
only short plants. Although he could not explain his
results at the time, Mendel realized that there must
be two kinds of tall pea plants: true-breeding plants
and plants that did not breed true.
Mendel then wondered what would happen if he
took pollen from a plant that produced only tall
plants (a true-breeding plant) and dusted it onto the
pistit of a short plant (another true-breeding plant).
To identify the different generations of plants, Mendel gave them different names. He called the first
two parent plants the parental generation, or P generation. He called the offspring of the P generation
the first filial (rrHr-ee-uhl) generation, or F1 generation. (The word filial comes from the Latin word
filius, which means son.) Mendel discovered that all
of the plants in the F1 generation were tall. There
were no short plants at alMt was as if the trait for
shortness from one of the parent plants had disappeardd completely. Mendel could not explain these
results either.
What happened next was even more of a mystery.
Mendel covered the tall plants of the F, generation
and allowed them to self-pollinate. That is, the pollen of a flower was allowed to fall onto the pistil of
the same flower. Mendel expected that the tall
plants would again produce only tall plants. But
once again he was surprised. Mendel discovered that
some of the plants in what he called the second filial
generation, or F2 generation, \Mere tall and some
were short. The trait for shortness seemed to have
reappeared! How could this have happened?
Figure 1-5 Mendel crossed tall
and short pea plants. He
discovered that the offspring in
the first generation were all tall.
What kind of plants were
produced in the second
generation?
t
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How Do You Measure Up?
1. Choose
a partner. Mea-
sure each other's height to
the nearest centimeter. Record
your measurements.
2. On the chalkboard, compile a list showing the height of
each student in your class,
3. Make a bar graph showing the number of students for
each recorded height measurement.
r Based on your graph, do
you think height in humans is
determined in the same way
as stem length in pea plants?
Give reasons for your answer.
Figure 1-6 To Mendel's surprise,
when he crossed two tall pea
plants from the F, generation, the
trait of shortness reappeared in
the F, generation. Why did the
shortness trait reappear?
x
Self-pollination
Tall plant (Tt)
Tall plant (TT)
Observing Traits
Visit a garden center or
greenhouse. Take a notebook
and a pencil to record your
observations.
1. Choose one type of flowering plant to observe, such
as petunias, marigolds, or
chrysanthemums.
2.To observe the genetic
traits of the flowering plant
you chose, you must observe
10 of these plants. Look closely at each of the plants.
3. Note common and un-
common traits among the
plants, such as the shape of
the leaves or the color of the
flower petals.
What common traits did you
observe on most of the plants?
What uncommon traits did
you see on one or more of the
plants but not on most of
them?
e How can you determine
which traits are dominant?
Tall plant (Tt)
Tall plant (Tt)
Short plant (tt)
From the careful records he kept of all his experiments, Mendel made several important discoveries.
He observed that the tall plants of the F1 generation
did not breed true. So he reasoned that these plants
had to contain factors for both tallness and shortness. When both factors were present in a plant,
only tallness showed. These factors, which Mendel
called "characters," are no\M.called genes. Genes are
the units of heredity.
ffiorminant and Recessive Traits
-"
From his observations, Mendel also concluded
that when he crossed two true-breeding plants with
opposite traits (tallness and shortness, for example),
the offspring plants showed only one of the traits
(tallness). That trait seemed to be "stronger" than
the other trait (shortness). The stronger trait is
called the dominant trait. The "weaker" trait, or the
trait that seemed to disappear, is called the recessive
trait.
Geneticist5-56i6ntists who study heredity-use
symbols to represent the different forms of a gene.
A dominant form is represented by u capital letter.
For example, the gene form for tallness in pea
plants is T. A recessive form is represented by u
small, or lowercase, letter. Thus, shortness is t. Every
organisrn has two forms of the gene for each trait.
So the symbol for a true-breeding tall plant is TT.
The symbol for a true-breeding short plant is tt.
Figure 1-7 All organisms, even
these rabbits, show a combination
of dominant and recessive traits.
How would you describe the traits
of each rabbit?
Dominant and Recessive
Traits
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1-1
Section Review
iF+,:,-ryr, :EE::rl
l.
What is genetics?
2. Compare dominant and recessive traits.
Inferences
Critical Thinking
-Making
3. When Mendel crossed pea plants that produced
only round seeds with plants that produced
only wrinkled seeds, all the plants in the F, generation produced round seeds. However, in the
F2 generation some plants produced wrinkled
seeds. Which t1211-1eund seeds or wrinkled
seeds-is dominant? Which is recessive? Explain
your answers.
1. Obtain two coins.
2. Cut four small, equalsized pieces of masking tape
to fit on the coins without overlapping the edges.
3. Place a piece of tape on
each side of both coins.
4. Write a capital letter T
on one side of each coin and
a lowercase letter t on the
other side.
5. Toss both coins together
100 times. Record the letters
of the genetic makeup for
each toss of the coins.
What are the possible gene
combinations? What is the
percentage of each?
.
*2
€
Principles
of Genetics
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'9
Seed
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Round
Yellow
Seed Coat
, ,Color,;"t:
for
Reading
Figure 1-8 The chart shows the
seven characteristics that Mendel
studied in pea plants. Each
characteristic has a dominant
gene and a recessive gene. Which
seed color is dominant in pea
plants?
PEA PLANT TRAITS
Color
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Focus on this question as
you read.
?> What are six basic
principles of genetics?
dominant one-appeared in the offspring. But in
the next generation, the trait that seemed to disappear-the recessive one-reappeared.
As an example, let's examine a cross between a
plant with yellow seeds (YY) and a plant with green
seeds (yy). The seeds that are produced in the F,
generation are all yellow, not a mixture of green and
yellow. Why? The gene for yellow seeds, Y, is dominant. It masks, or hides, the recessive gene for green
seeds, y. Therefore, all the seeds are yellow. What
happens when a plant from the F, generation pollinates itself? In this case, most of the seeds are
Seed
Shape:
i. ".,: 't:,.i. ":i.:-,.
Guide
As you read in the previous section, one of the
reasons Mendel chose pea plants for his experiments
was that they showed a variety of different traits that
could be studied at the same time. So in addition to
height, or stem length, Mendel also studied seed
shape, seed color, seed coat color, pod shape, pod
color, and flower position. These traits are illustrated in Figure 1-8. For every trait studied, the
results were always the same: Crossing two truebreeding plants with opposite traits did not result in
a mixture of the traits. Only one of the traits-the
,
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.,.,, Pr-od .t...':-... , ,
'rtShape,'iir,.
, :.;;.:;,
Colored
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#
Wrinkled
Green
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White
Pinched
,i
I
u]"
.1
.1fr
'1
t
/
//
.Yellow
Short
yellow, but some are green. The recessive gene for
green seeds reaPpears in the F, generation.
An organism that has genes that are alike for a
particular trait, such as YY or yy, is called a pureLred. An organism that has genes that are different
for a trait, such as Yy, is called a hybrid (nrcrrbrihd). The plants with yellow seeds in Mendel's F1
generation were hybrid plants (Yy). They were Produced by crossing two purebred plants with opposite
traits (YY and yy). Many of the plants advertised in
seed catalogs are hybrids that were developed by
plant breeders. You will learn more about plant and
animal hybrids in ChaPter 4-
l-aws of Genetics
After performing hundreds of experiments and
Figure 1-9 Mendel discovered
that a pea plant with green seeds
can develop from a cross
between parents with Yellow
seeds. How does this haPPen?
Figure 1-10 These colorful
ziinias (left) and lovelY PurPle
petunias and rose-colored vincas
(right) are hybrids that were
produced by Plant breeders.
analyzing his observations, Mendel formed a hypothesis about how traits were passed on from one
generation of pea plants to another. (Remember, a
hypothesis is a suggested explanation for a scientific
probl"*.) Mendel's hypothesis was that each pea
pturrt had a pair of factors, or genes, for each trait'
Each parent pea plant could contribute only one
gene of each pair to each plant in the next generaiion. In that way, each plant in the next generation
also had.a pair of.genes for each trait, one from
Now Mendel could account for the fact that a
pea plant with green seeds can develop from a cross
between parents with yellow seeds. The factor, or
gene, for the green color must be present but hidden in the parents. For example, a parent with Yy
genes would have yellow seeds because the dominant
gene (Y) was present. But that parent would also be
carrying the recessive gene (y) for green seeds. The
green seed trait would be hidden in the parent but
could be passed to its offspring.
When the parent plant forms sex cells (sperm or
eggs), the parent's gene pairs segregate, or separate.
This process is known as the law of segregation.
According to the law of segregation, o_ne gene from
each pair goes to each sex cell. Half of the sex cells
of a hybrid pea plant with the gene pair Yy have a
gene for yellow seeds (Y). The other half of the sex
cells carry a gene for green seeds (y). As a result of
sexual reproduction, a male sex cell (sperm) and a
female sex cell (.gg) unite to form a fertilized egg.
Each fertilized egg contains one gene for seed color
from each parent, so the gene pair for seed color is
formed again.
Mendel also crossed pea plants that differed from
one another by two or more traits. The results of
these crosses led to the law of independent assortment. The law of independent assortment states that
each gene pair for a trait is inherited independently
of the gene pairs for all other traits. For example,
when a tall plant with yellow seeds forms sex cells,
the genes for stem length separate independently
from the genes for seed color.
Figure 1-11 The diagram
illustrates the law of segregation,
describing how one of the
parent's genes goes to each sex
cell. Notice how the baby
rhinoceros and the sheepdog
puppies resemble their parents.
Why do organisms resemble their
parents?
Yy
/
Sex cell
\
Parent
"y
Sex cell
LAW OF SEGREGATION
BnconnPlete Dominan(e
or
Mendel's idea that genes are always dominant
in most cases but
always recessive p'ou"J to be true
Correns
rrct ;U. In 1900, ttre German botanist Karl that in
found
made an important discovery' Correns
dominant nor
;;; gene pairs, the genes are neither
incomplete
recessive- Instead, theie genes show
that neidominance. Incomplete dominance means
As a result'
ther gene in a gent pait masks the other'
in. tiuirs carri."d Uy tfre two genes appear to be
blended.
Corrensdidmuchofhisworkwithatypeof
l-Lz'
Fioure 1'12 ln four'o'clock
flo'wers, neither the red gene nor
the white gene for flower color is
dominant. When these two genes
are oresent in the same Plant, a
oink flower results' What kind of
'inheritance does this show?
ptunt called a four-o'c1ock, shown in Figure
he discovered that when he crossed purebred red
fouriorl.-o'.Iock flowers (RR) with purebred ryhritefourpink
o'clock flowers (WW), the result was all
for red color
o'clock flowers inW Neither the gene
Neither
,"t ,t. gene for white color was dominant'
the two
g.rr" in ihe gene pair was rnasked" Instead' to
t"ruit, for flower Jolor, red and white, seemed
resulted in
have blended together' This blending
pink flowers. NJtice that in the case of incomplete
"a"*r""".L, th" symbols for the gene pairs for red
are all capiflowers, white flowers, and piTk flowers
for red
tal letters. This is because neither the gene
floweisnorthe-geneforwhiteflowersis.-dominant
Fiqure 1-13 Traits-such as the
,itor, markings, tail length, and fur
t"ngin in these cats-are Passed
on"from one generation of
organisms to the next.
over the other.
Incomplete dominance also occurs in animals'
is seen in
One of the most famous examples of this
the beautifut horses known as palominos' Palominos
If
ur. prt. golden-brown with a white mane and tail'
a purebred chestnut-brown horse (BB) is crossed
with a purebred creamy-white horse (WW), their offspring will all be palominos (BW). If two palominos
are crossed, what colors will their offspring be? Is it
possible to have purebred palominos?
Genetic Principles
Through the work of scientists such as Mendel
and Correns, certain basic principles of genetics
have been established. These basic principles are
r
r
r
r
r
r
Traits, or characteristics, are passed on from one
generation of organisms to the next generation.
The traits of an organism are controlled by
genes.
Organisms inherit genes in pairs, one gene
from each parent.
Some genes are dominant, whereas other genes
are recessive.
Dominant genes hide recessive genes when both
are inherited by an organism.
Some genes are neither dominant nor recessive.
These genes show incomplete dominance.
In 1866, Mendel published
a paper describing his
experiments and conclusions in a little-known scientific journal. Scientists at that time, however, were
not prepared to accept Mendel's results. They did
not understand the importance of his work. Mendel's paper remained unread and unappreciated for
many years. Finally, in the early 1900s, Mendel's
paper was rediscovered. Scientists realized that Mendel had correctly described the basic principles of
genetics. Mendel's discoveries were at last recognized
as an important scientific breakthrough.
1-2 Section Review
t. List six basic principles of genetics.
c,
What is incomplete dominance?
3. What is a hybrid organism?
Critical Thinking
Concepts
-Relating
4. Can a short-stemmed
pea plant ever be a hybrid? Explain why or why not.
lncomplete Dominance
ln this activity, you will use
coins to model a cross between two plants with the
gene pair RW.
1. Obtain two coins.
2. Cut four equal pieces of
masking tape to fit on the
coins without overlapping.
3. Place a piece of tape on
each side of both coins.
4. Write the letter R on one
side of each coin and the letter
W on the other side.
5.Toss both coins 100
times and record the gene
pair for each toss.
What is the percentage of
occurrence for each gene
pai?
Guide for Reading
Focus on this question as
you read.
) How can you predict the
results of genetic crosses
using probability and
Punnett squares?
1-3 Genetics and Probability
fn one of Mendel's experiments, he crossed two
plants that were hybrid for yellow seeds (Vy). When
he examined the plants that resulted, he discovered
that about one seed out of every four was green. By
applying the concept of probability to his work,
Mendel was able to express his observations mathematically. He could say that the probability of such a
cross producing green seeds was L/4, or 25 percent.
Probability is the possibility, or likelihood, that a
particular event will take place. Probability can be
used to predict the results of genetic crosses.
Probability
Flip Outl, p.'105
Figure 1-14 According to the
law of probability, a coin will land
heads up 50 percent of the time
and tails up 50 percent of the
time. What is the probability that
the next child born to these
parents, who already have four
sons, will be a boy? A girl?
Suppose that you are about to toss a coin. What
. are the chances that the coin will land heads up? If
you said a 5O-percent chance, you are correct. What
are the chances that the coin will land tails up?
Again, the answer is 50 percent. Atthough you may
not realize it, you, like Gregor Mendel, used the laws
of probability to arrive at your answers. You figured
out the chance, or likelihood,,that the coin would 4come up heads (or tails) on one toss.
A probability is usually written as a frdction or as
a percdntage. For example, the chance that a sex cell
will receive a Y gene from a parent with a Yy gene
pair is 1/2, or 50 percent. In other words, you
would expect one half, or 50 percent, of the sex
cells to receive a Y gene
In probability, the results of one event do not
affect the results of the next. Previous events do not
affect future outcomes. Each event happens independently. For example, suppose you toss a coin 10
times and it lands heads up each time. What is the
probability that it will land heads up on the next
toss? Because the coin landed heads up on the previous 10 tosses, you might think that it is also likely to
land heads up on the next toss. But this is not the
case. The probability of the coin's landing heads up
on the next toss is still 1/2, or 50 percent. The
results of the first 10 tosses do not influence the
result of the eleventh toss.
Punnett Squares
In addition to probability, a special chart called
a Punnett square is used to show the possible gene
combinations in a cross between two organisms.
This chart was developed by Reginald C. Punnett, an
English geneticist.
Let's see how a Punnett square works. Look at
the Punnett square in Figure l-15. It shows a cross
between two guinea pigs. The two possible genes in
the fernale sex cells are listed across the top of the
chart. The two possible genes in the male sex cells
are listed along the left side. Remember, when a
male sex cell (sperm) and a female sex cell (.SS) join,
a fertllized egg forms. Each box in the Punnett
square represents a possible gene pair in the fertilized egg
Notice that in the P (parent) generation, both of
the female's gbnes are for black hair (BB). Both of
the male's genes are for white hair (bb). All of the
offspring in the F, generation are hybrid black (Bb).
If you were to look at these hybrid black-haired
guinea pigs, you would not be able to tell the difference between them and purebred black-haired
guinea pigs. Their phenotypes (rnr-noh-tighps), or
physical appearances, are the same. Phenotype refers
to a visible characteristic; in this case, black hair.
However, their genotypes .(JrHN-uh-tighps) are
different. A genotype is the actual gene makeup of
en orsanisrn
bb,
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r: :t ,::,_i/ti ",r. r\"
,i
1.
@@
@@
Female sex cells
BB
a
6o
xo
o
o
E
Figure 1-15 This Punnett square
shows a cross between a black
guinea pig and a white guinea pig.
What is the phenotype of the
offspring?
Probability
What is the probability that
two plants with the genotype
Rr, where R = srnooth seeds
and r = wrinkled seeds, will
produce an offspring that
has wrinkled seeds? Express
your answer as a fraction
and as a percentage.
Fioure 1-16 This Punnett square
sh'ows a cross between two
hybrid btack guinea Pig?.. W.l't
fhenotypes are Present in the
offspring?
Fm XM
@@
@@
Female sex cells
B
b-
A purebred black-haired guinea pig ha1 the geno-
lig; . hybrid
black-haired guinea pig has the
g.rro,yp. Bb. Both the purebred guinea pig and the
tybrid^guinea pig have black hair' Figure 1-16
,ho*r,u".ros b"t*...r two of the hybrid black-haired
guinea pigs from the F1 generation' The results of
the F, cross, or the F, generation, are L/4 purebred
black (BB), l/2 hybrid tlack (Bb), and L/4 purebred
white iUnl. Wn"t percentage of the offspring have
the same genotype as the parents? What percentage
have the same PhenotYPe?'.
type
1-3
.6
Section Review
a
l.
o
tr6
l
o
6
-b
\l'."
',,,,,,-a,.j"'t'
bb
What is ProbabilitY?
2. What is ihe difference between genotype and
phenotYPe? Give an examPle'
Critical Thinking -Making Predictions
3. Use a Punnett square to predict the outcome of
a cross between i ftyUtia black guinea pig and a
white guinea pig. What are the possible genotypes Jf tfr. offipring? What are the possible
(Hint: Whut is the gene makeup of
ffi.tto,ypes?
ihe white guinea Pig?)
using punnett squares to solve Genetics Problems
One-Factor Crosses: SamPle
Problem
ln pea plants, round seeds are domi-
nant over wrinkled seeds. Predict the
genotypes and phenotypes of a cross
between two hybrid round-seeded pea
plants.
Step
'l Choose a letter to represent
the genes in the cross.
R=
rouFld
r = wrinkled
Use a letter whose capital form does
not look too similar to its lowercase
form. This will make it easier for you to
read your finished Punnett square. Except for that requirement, it is not important which letter you select. ln this
case, R is the dominant round gene and
r is the recessive wrinkled gene.
Step 2 Write the genotypes of the
parents.
nrxnr
This step is often written as an abbreviation of the cross being studied. The x
between the parents' genotypes is read
This step represents the process of
fertilization. The gene from the top of
the box and the gene from the side of
the box are combined inside each of the
four boxes. lf there is a combination of
capital letter and lowercase letter in a
box, write the capital letter f irst. The letters inside the box represent the probable genotypes of the offspring resulting from the cross. ln this example, 1/4
of the offspring are genotype RR, 112
are Rr, and '1 14 are rr.
Step 6 Determine the phenotypes
of the offspring.
'ui.R,,,
"is crossed with." ln this case, Rr is
crossed with Rr.
:R
Step
3 Determine the possible
genes that each parent can
produce
Rr
Rr
/\
/ \
/
RrRr
Step
/\
\
4 Write the possible genes at
the top and side of the Punnett square.
R.,,
r
r
RR
o s
Rr
rr
Remember that phenotype refers to
the physical appearance of an organism. The principle of dominance makes
it possible to determine the phehotype
that corresponds to each genotype inside the Punnett square. ln this example, 314 o't the offspring have round
seeds and 'l 14 of the offspring have
wrinkled seeds.
D
ll
r
Step 5
Complete the Punnett
square by writing the gene
combinations in the appropriate boxes.
'i,,R
*
RR
.--->
l*,.
r
,
':f
,
{
Rr
,.,.J
One-Factor Crosses: Practice
Problems
1. What are the possible genotypes
and phenotypes produced when a hybrid round-seeded pea plant (Rr) is
crossed with a purebred wrinkledseeded plant (rQ?
2. What are the possible genotypes
and phenotypes produced when
a
purebred round-seeded pea plant (RR)
is crossed with a hybrid round-seeded
plant (Rr)?